Inulin is a naturally occurring fructo-oligosaccharide composed of a mixture of oligomers of varying degrees of polymerization ("DP") or molecular weights that occurs naturally plants such as onion, garlic, Jerusalem artichoke, dahlia and chicory for plant energy storage. The inulin produced from different plants, at different stages in the growing cycle of a plant, or under different climatic conditions, will normally have different average DP's.
One of the limitations that face the industry now, namely, that an entire crop of roots must be harvested and processed within 2 months to get the inulin before it is broken down to fructose. The present situation (using the prior art techniques) requires a large facility to process a larger quantity of material in a short time, which prevents effective use of economy of scale because the processing facility would lie idle for much of the year.
In Europe, chicory is used as the source for inulin. In the prior art, inulin is extracted from the chicory roots by soaking the sliced vegetable (cossetts) in hot water, or macerating the roots, then pasteurizing the mash, and filtering off the extract. The resulting extract contains a complex mixture of variously sized chain lengths of fructose linked .beta. (2.fwdarw.1) with, occasionally, an .alpha.-D-glucopyranosyl residue at the reducing end of the chain, along with fructose, glucose, sucrose, salts, fats, proteins and amino acids. Heating the mash is considered essential to inactivate inulin-degrading enzymes (inulinases). Proteins and other polar components are then removed by treatment with lime, and/or carbon and diatomaceous earths, then the carbohydrate stream is deionized with ion exchange resins. When a high molecular weight (MW) or large DP inulin fraction is desired, it is typically isolated by ethanol precipitation, crystallization, chromatography or ultrafiltration. These methods were employed to reduce the content of mono- and disaccharides, salts or amino acids at the low MW range, and to reduce the content of proteins, cellulose fibers and other debris at the high MW end. Where ultrafiltration has been applied in the prior art, it was to do a single separation by removing lower MW components as the membrane permeate while retaining the larger MW inulin. For example, Berghofer et al. (cited below) have used ultrafiltration to remove non-inulin components using hollow fiber cartridges with molecular weight cut-off of 2000 or 5000 (Romicon PM2 or PM5 respectively), but this method results in the loss of more than half of the inulin with no evident fractionation.
Where low DP fructooligosaccharides are the desired product, industry today uses either acids or enzymes to break down the high MW fractions to achieve a common quality composition. This requires additional processing to that outlined above to effect the hydrolysis, then remove the enzyme or mineral acid thereby adding to the overall process cost.
The present invention provides a process that obviates the necessity to achieve uniform quality inulin products by the use of techniques to break down the high MW fractions. By taking advantage of the natural distribution of compositions, the present invention also does not restrict recovery of inulin from natural products to a narrow time of harvesting and processing. The processing time can then advantageously be extended over a longer period of time, and thereby allow for gradual processing of the harvest in a smaller facility which is operated continuously all year. As a result, a smaller facility can be employed that doesn't lie idle for a significant proportion of the time. Although the present process can use inulin from any of the commonly available sources, Jerusalem artichoke is better suited for North American agriculture (climate, etc.,).
A further contribution of this invention is the use of membrane filtration to clarify the extract, thereby rendering unnecessary the use of lime and carbonation, or filtration using filtration aids (such as diatomaceous or siliceous earths), and to use a series of membranes with discrete MW cut-off ranges to generate a family or series of purified inulin products useful for human use as food or in therapeutics.
These products comprise a series of fractions having relatively narrow DP ranges, which have different properties that allow them to function in distinct capacities in food systems. For example, the higher DP ranges may better serve as thickeners and/or fat replacers, whereas the lowest DP range are known to have properties in food systems resembling those of sugar (sucrose). Intermediate ranges are not presently available commercially, but are expected to be more like sugar than the larger DP ranges, but to lend more thickening than the low DP ranges such as are obtained by hydrolysis.
Since the average MW of inulin in Jerusalem artichoke tubers (and other sources such as chicory and dahlia tubers) is known to vary with time of harvest (lower MW being favored in later harvested tubers), fractionation and blending can serve to provide a consistent product and permit formulation of "custom" blends for specific applications. Blending allows consistency of composition for any given product, so that the properties are consistent and predictable--this is very important in food processing so that formulation can be the same from batch to batch. Blending also allows preparation of a variety of products having different MW profiles.